The invention relates to hydrogen seal rings and, more specifically, to a seal structure for automatically resetting in the event of oil pressure loss and subsequent restoration, by reducing oil leakage through a circumferential gap between the seal ring segments and seal structure in a hydrogen cooled generator.
Hydrogen has been widely used as a coolant in a variety of rotary dynamoelectric machines, due to its desirable thermophysical properties including low density and high specific heat. However, a primary disadvantage of using hydrogen is that the hydrogen purity must be maintained above its explosive limit (74% hydrogen in air). Therefore, a primary consideration for ensuring the safe operation of hydrogen-cooled rotary machines, such as turbo-generators, is designing highly reliable and efficient hydrogen seal systems therefor.
In a hydrogen-cooled generator, hydrogen seals are utilized both to seal high-pressure hydrogen at the interface of the rotating shaft, and to prevent air from entering the casing and developing an explosive mixture with the hydrogen. Before the early 1980s, hydrogen seal systems consisted of a pair of four segmented bronze rings disposed in a seal casing. The newer babbitted steel seal rings 10 are each made in two 180° segments 12, 14 as illustrated in FIG. 1. A typical hydrogen seal system is schematically shown in FIG. 2. In that illustration, an annular seal casing is partially shown which is adapted to be mounted to a generator end shield (not shown) in surrounding and sealing relationship with a rotor/shaft 16. The casing is formed in two main parts, referred to hereinbelow as casing halves, each extending 180° about the shaft. For ease of description, the upper casing half 18 and the seal ring segments 12 disposed therein are illustrated and will be described in detail. It is to be understood, however, in an exemplary embodiment, the lower casing half has a corresponding construction. The upper casing half 18 is of two-part construction, including a seal casing main body 20 and a seal casing cap segment 22. The seal casing cap segment has a generally h-shaped cross-section, forming a radially inwardly directed chamber 24 opening in a radially inward direction towards the shaft 16 for housing radially inwardly projecting seal rings 12 which in turn engage the shaft.
Each seal casing cap segment 22 is formed with an axial portion 26 connecting an upper radial flange portion 28 and lower inner radial portion 30 and outer radial portion 32. The axial portion 26 thus defines a base for the chamber 24 while radial portions 30 and 32 form two, axially spaced, parallel sides of the chamber 24. Axially opposed shoulders 34, 36 define an opening facing the rotor shaft 16. The seal casing cap segment 22 is fastenable to the seal casing main body 20 by a semi-annular array of bolts 38 passing through holes in the radial flange portion 28 of the cap and threadably received in the main body 20.
Within the chamber 24, there are seated a pair of side-by-side seal ring segments 12, each extending approximately 180° about the casing half 18. The rings 10 are held together radially and apart axially by two coil springs 40 (only one of which is shown in FIG. 2), each extending substantially 180° within the chamber 24. The spring is seated within an area created by tapered surfaces 42 on the respective ring segments 12. Opposite ends of the spring are anchored to axially extending pins (not shown) via a hook or the like (not shown) formed at each end thereof. As is conventional, the pin is located within aligned bores in radial wall portions of the casing cap segment. The pin is also used to align and hold a labyrinth type oil seal 46. The spring biases the seal ring segments 12 radially inwardly and in axially opposite directions, against opposed faces of the inner and outer radial wall portions 30, 32 of the chamber 24.
In use, seal oil is introduced into the cavity of chamber 24 behind or radially outside the seal rings 12, at a pressure higher than the hydrogen pressure inside the casing. Then, the high pressure seal oil flows radially between the seal rings 12 toward the rotating shaft 16, where the sealing oil flow divides and runs axially with the clearance between the shaft and seal rings. At the hydrogen side 48 of the seal rings, the oil flows evenly between the shaft and the inner seal ring all the way around the seal ring at their interface and thus seals hydrogen from leaking and keeps the seal ring centered on the shaft. Similarly, the oil is uniformly distributed between the shaft 16 and the outer seal ring at the air side 50 of the seal.
As illustrated in FIG. 1, hydrogen seal rings 10 are usually made into segments 12, 14, split at horizontal joints. The two segments can either be bolted together at the horizontal joint or held by two coil springs suitably attached to the casing. As noted above, the purpose of the hydrogen seal springs 40 is to separate the two sealing rings and keep the sides of the rings against the casing. In normal operation, these rings maintain a uniform clearance and do not allow oil leakage at the ring segment joints. They are free to expand radially but prevented from rotating by either the pins to which the springs are attached or an anti-rotation device. In this way, the rings can float freely with respect to the seal casing cap 22.
However, under certain circumstances, the seal rings 12 may stick together so that oil ceases to flow between the seal rings. One possible cause is a loss in seal oil pressure. If the seal oil pressure drops to an undesirably low level the inner seal oil ring may move towards the outer seal ring until their axial surfaces make contact. This blocks the oil flow channel and the seal rings can be resistant to separating due to static friction between the two axial surfaces. Another problem is that when both seal rings contact each other in this manner, a large channel is opened up between the shoulder 34 and the opposing surface of the seal ring 12. This channel can allow oil to flow into the generator in undesirable quantities and possibly result in a forced shutdown of the generator.